Spinal proper functions:

1. Spinal reflexes.

2. Organism motor activity co-ordination particularly spinal motoneurons functions.

3. Vegetative reactions.

4. Uroreleasing and defecation.

Spinal animals – are animals with cutted spine on the boarder with medulla oblongata.

Spinal animals features:

1. Respiration absence.

2. Low blood pressure, decreased vascular tone.

3. Loosing the ability to support homoiothermism (constant body temperature).

4. Disapperance of all forms of singleminded activity (alimentary, sexual, protective).

5. Anal and uroreleasing ceneters paralysis at spine cutting lower than lumbal parts.

Spine vegetative functions:

1. Defecation.

2. Uroreleasing.

3. Vascular tone regulation.

4. Erection and ejaculation.

5. Sweat releasing.

Spine afferent ways:

1. Goll’s and Burdah’s ways.

2. Spinal-thalamic tracts.

3. Spinal-cerebellar tracts.

Significance of afferent information coming into spine:

1. It carries the information about environment changings.

2. It participates in CNS co-ordinative activity by guiding the skeletal musculature: at switching-off the afferent impulsation from working organ its guiding becomes non-perfect.

3. It encourages CNS tone supporting; summary tonic CNS impulsation is decreased at afferent impulsation switching off.

4. It participates in inner (visceral) organs regulation processes.

Efferent ways:

1. Pyramidal tract.

2. Extrapyramidal ways:

   1. reticulo-spinal tract;

   2. rubro-spinal tract;

   3. tecto-spinal tract;

   4. vestibule-spinal tract.

Pyramidal tracts

Pyramidal tracts of spinal cord are descending tracts concerned with voluntary motor activities of the body. These tracts are otherwise known as corticospinal tracts. There are two corticospinal tracts, the anterior corticospinal tract and lateral corticospinal tract. While running from cerebral cortex towards spinal cord, the fibers of these two tracts give the appearance of a pyramid on the upper part of anterior surface of medulla oblongata. Hence, these two tracts are called pyramidal tracts.

The pyramidal tracts are concerned with voluntary motor activities and were the first tracts to be found in man (Fig. 20).

Fig. 20. Corticospinal tract.

Nerve fibers of pyramidal tracts: All the fibers of the pyramidal tracts are present since birth. However, the myelination of these fibers is completed in about two years after birth. The pyramidal tracts on each side have more than a million fibers. About 70% of the fibers are myelinated and large having a diameter of 4 to 22 microns.

Larger fibers of pyramidal tracts have the tendency to disappear at old age. Since these tracts are concerned with control of voluntary movements, the disappearance of the fibers of pyramidal tracts causes automatic shivering movements in old age.

The fibers of pyramidal tracts are the axons of upper motor neurons.

Origin

Fibers of pyramidal tracts arise from the following nerve cells in the cerebral cortex.

1. Giant cells or Betz cells or pyramidal cells in precentral gyrus of the motor cortex. The giant cells are situated in area 4 (primary motor area) of frontal lobe of the cerebral cortex.

2. Other areas of motor cortex namely, premotor area (area 6) and supplementary motor areas.

3. Other parts of frontal lobe.

4. Parietal lobe of cerebral cortex particularly from somatosensory areas (areas 3,1,2). It is believed that 30% of pyramidal fibers arise from primary motor area (area 4) and supplementary motor areas, another 30% from premotor area (area 6) and the remaining 40% of fibers arise from the parietal lobe particularly from somatosensory areas (areas 3,1,2). All the above fibers form the fibers of upper motor neurons of motor pathway.

Course

After taking origin, the nerve fibers run downwards in a diffused manner through white matter of cerebral hemisphere and converge in the form of a fan like structure along with ascending fibers which project from thalamus to cerebral cortex. The fan like structure is called corona radiata. Then, the fibers descend down through internal capsule, midbrain and pons. While descending through pons, the fibers are divided into different bundles by the nuclei of pons. At the lower border of pons, the fibers are grouped once again into a compact bundle and then descend down into medulla oblongata.

This compact bundle of corticospinal fibers gives the appearance of a pyramid in the anterior surface of upper part of medulla. Because of this, the corticospinal tracts are called the pyramidal tracts.

At the lower border of medulla, the pyramidal tract on each side is divided into two bundles of unequal sizes. About 80% of fibers from each side cross to the opposite side. Thus, the fibers of both sides while crossing the midline form pyramidal decussation. After crossing and forming pyramidal decussation, the fibers descend through the posterior part of lateral white funiculus of the spinal cord. This bundle of crossed fibers is called the crossed pyramidal tract or lateral corticospinal tract or indirect corticospinal tract.

The remaining 20% of fibers do not cross to the opposite side but descend down through the anterior white funiculus of the spinal cord. This bundle of uncrossed fibers is called the uncrossed pyramidal tract or anterior corticospinal tract or direct corticospinal tract. This tract is well-marked in cervical region. Since, the fibers of this tract terminate in different segments of spinal cord, this tract usually gets thinner while descending through the successive segments of spinal cord. The fibers of this tract are absent mostly below the midle thoracic level. Before termination, majority of the fibers cross to the opposite side at different levels of spinal cord.